This paper describes an approach to harvesting electrical energy from a mechanically excited piezoelectric element. A vibrating piezoelectric device differs from a typical electrical power source in that it has a capacitive rather than inductive source impedance, and may be driven by mechanical vibrations of varying amplitude. An analytical expression for the optimal power flow from a rectified piezoelectric device is derived, and an “energy harvesting” circuit is proposed which can achieve this optimal power flow.

The harvesting circuit consists of an ac–dc rectifier with an output capacitor, an electrochemical battery, and a switch-mode dc–dc converter that controls the energy flow into the battery. An adaptive control technique for the dc–dc converter is used to continuously implement the optimal power transfer theory and maximize the power stored by the battery. Experimental results reveal that use of the adaptive dc–dc converter increases power transfer by over 400% as compared to when the dc–dc converter is not used.

INTRODUCTION

The need for a wireless electrical power supply has spurred an interest in piezoelectric energy harvesting, or theextraction of electrical energy using a vibrating piezoelectric device. Examples of applications that would benefit fromsuch a supply are a capacitively tuned vibration absorber [1], a foot-powered radio “tag” [2], [3], and a PicoRadio [4].

A vibrating piezoelectric device differs from a typical electrical power source in that its internal impedance is capacitive ratherthan inductive in nature, and that it may be driven by mechanical vibrations of varying amplitude and frequency. While there havebeen previous approaches to harvesting energy generated by a piezoelectric device [2], [3], [5], [6] there has not been an attemptto develop an adaptive circuit that maximizes power transfer from the piezoelectric device.

OPTIMAL POWER FLOW OF PIEZOELECTRIC DEVICE

To determine its power flowcharacteristics, a vibrating piezoelectric element is modeled as a sinusoidal current sourcein parallel with its internal electrode capacitance . This model will be validated in a later section. The magnitude of the polarizationcurrent varies with the mechanical excitation level of the piezoelectric element, but is assumed to be relatively constantregardless of external loading. A vibrating piezoelectric device generates an ac voltage while electrochemical batteriesrequire a dc voltage, hence the first stage needed in an energy harvesting circuit is an ac–dc rectifier connected to the outputof the piezoelectric device, as shown in Fig. 1. In the following analysis, the dc filter capacitor is assumed to be large enough so that the output voltageis essentially constant; the load is modeled as a constant current source ; and the diodes are assumed to exhibit ideal behavior.